Servlet/JSP-03 HttpServlet

一. GenericServlet

　　GenericServlet本身是一个抽象类，并且实现了Servlet和ServletConfig接口

　　其在内部定义了一个私有的ServletConfig类型的变量config，并在init(ServletConfig config)为其赋值，然后通过config实现其他方法。

　　service(ServletRequest servletrequest, ServletResponse servletresponse)为抽象方法。具体代码如下：

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable
{
    private transient ServletConfig config;

    public GenericServlet() { }

　　 @override
    public void init(ServletConfig config) throws ServletException
    {
        this.config = config;
        init();
    }

    // 此方法供我们实现的Servlet初始化时使用
    public void init() throws ServletException {}

    @override
    public ServletConfig getServletConfig()
    {
        return config;
    }
    
    @override
    public String getInitParameter(String name)
    {
        return getServletConfig().getInitParameter(name);
    }

    public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)
        throws ServletException, IOException;

    // 其他方法略
    ...

}

二. HttpServlet

为了简化开发，Servlet容器提供了HttpServlet的实现，其继承了GenericServlet.

1. Servlet容器为我们封装了HttpServletRequest, HttpServletResponse类型的参数，传入service方法，在service方法内强转后传入对应的doXXX方法

2. 开发时继承HttpServlet，只需要重写doXXX方法即可，处理相应方式的HTTP请求。

public abstract class HttpServlet extends GenericServlet
{
    // doXXX方法被子类覆盖
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_get_not_supported");
        if(protocol.endsWith("1.1"))
            resp.sendError(405, msg);
        else
            resp.sendError(400, msg);
    }

    //doPost, doPut,  doDelete类似实现， doHead, doTrace, doOption略复杂
    ......

    //通过此方法调用不同HTTP请求方式的处理方法doXXX
    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();
        if(method.equals("GET"))
        {
            long lastModified = getLastModified(req);
            if(lastModified == -1L)
            {
                doGet(req, resp);
            } else
            {
                long ifModifiedSince;
                try
                {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                }
                catch(IllegalArgumentException iae)
                {
                    ifModifiedSince = -1L;
                }
                if(ifModifiedSince < (lastModified / 1000L) * 1000L)
                {
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else
                {
                    resp.setStatus(304);
                }
            }
        } else
        if(method.equals("HEAD"))
        {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else
        if(method.equals("POST"))
            doPost(req, resp);
        else
        if(method.equals("PUT"))
            doPut(req, resp);
        else
        if(method.equals("DELETE"))
            doDelete(req, resp);
        else
        if(method.equals("OPTIONS"))
            doOptions(req, resp);
        else
        if(method.equals("TRACE"))
        {
            doTrace(req, resp);
        } else
        {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object errArgs[] = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }
    }

    //此为GenericServlet中方法，最终调用HttpServlet内实现的service方法
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest request;
        HttpServletResponse response;
        try
        {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        }
        catch(ClassCastException e)
        {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
    }

}